1. Field of the Invention
Embodiments of the specification generally relate to satellite navigation receivers and more specifically to high sensitivity, GPS-assisted time sources.
2. Description of the Related Art
Satellite navigation systems allow electronic receivers to determine navigational information such as position (latitude, longitude, and altitude), velocity and time. One example of such a system is the United States Navstar Global Positioning System (GPS), which may include up to thirty functional navigation satellites. Other examples of satellite navigation systems include the Russian GLONASS system and the European Galileo system. Satellite navigation receivers, such as GPS receivers typically use GPS data from three or more orbiting satellites to determine navigation information. Only a portion of the satellites within a navigation system may be visible to a particular navigation receiver at a given time.
GPS satellites typically transmit GPS signals on two bands: the L1 band with a carrier frequency of approximately 1575.42 MHz and the L2 band with a carrier frequency of approximately 1227.40 MHz. Traditionally, only authorized users have been able to use data transmitted on the L2 band. In the future, civilian GPS signals may be transmitted on the L2 band and the L5 band (approximately 1176.45 MHz). Typically, low cost GPS receivers receive only on one of these bands. Some civilian GPS receivers may use clock data from the L2 band to refine GPS data carried in the L1 band. The following descriptions use the L1 band to describe exemplary embodiments; however, other embodiments may be implemented using one or more GPS bands or other global positioning signals.
GPS satellites transmit data using a form of spread spectrum coding known as code division multiple access (CDMA). Each satellite may be assigned a coarse acquisition (CA) code that resembles pseudo random noise and is typically unique to that satellite. Each satellite encodes data using the satellite's CA code and transmits encoded data on the L1 carrier frequency (i.e., data is spread using the CA code). Thus, all satellites are simultaneously transmitting data on a shared carrier frequency. Each CA code consists of a sequence of 1023 “chips” where each chip may be assigned a value of one or zero. The CA code is transmitted at a rate of 1.023 MHz; therefore, each chip period is approximately 0.977 us. In some embodiments, a ground-based pseudo-GPS satellite (i.e., a pseudo-lite) may transmit GPS data by using a CA code of a satellite that may be out of view of the GPS receiver. CA code phase is the relationship of a CA code either to a reference clock or to other CA codes transmitted by other satellites. Although the CA code phase may be synchronized between satellites at the time of transmission, the CA codes may be received with differing delays at the GPS receiver due to different and changing propagation times. Typically, a GPS receiver determines which CA codes are being received in order to determine which GPS satellites are in view. Once a GPS signal with a particular CA code is received and identified, the GPS receiver is said to have “acquired” the GPS satellite associated with that CA code. A GPS receiver may also “track” a GPS satellite by continuing to receive a GPS signal from a previously acquired GPS satellite.
Typically, after a GPS receiver acquires four or more GPS satellites, the GPS receiver may determine a position, velocity, and time (PVT) solution. If the GPS receiver can acquire more than four GPS satellites, the PVT solution may be made more accurate. Some GPS receivers may determine a PVT solution with less than four GPS satellites. Such a solution, however, may not be as accurate as a solution determined with four or more GPS satellites.
GPS satellites use relatively low power radio transmitters. For example, a typical GPS satellite may only be rated to 50 W. A typical orbit of a GPS satellite is approximately 14, 500 miles above Earth. At that distance, the signal strength of a GPS signal on Earth may be as low as −160 dBm. This signal strength may be well below the thermal noise floor of a GPS receiver. Well-known digital signal processing techniques are typically used to increase signal strength and recover transmitted data from a received GPS signal. Since the GPS signals are relatively weak, receiving GPS signals indoors may be difficult. For example, it may be difficult for GPS signals to penetrate the roof and walls of buildings. Without usable GPS signals, it may be too difficult for a GPS receiver to determine a relatively accurate PVT solution indoors.
Since four or more GPS satellites are typically required to provide a PVT solution, typical GPS receivers generally include multiple acquisition and tracking channels to provide a PVT solution relatively quickly. For example, multiple acquisition channels may acquire a GPS satellite more quickly than a single acquisition channel by distributing the acquisition task across multiple acquisition channels. Similarly, multiple tracking channels may be used to track a plurality of GPS satellites. Because of the nature of the GPS signals, oftentimes a particular GPS signal may fade and reception may be temporarily be lost when the GPS receiver is in motion. By tracking multiple satellites, the loss of a single GPS satellite signal may not hinder the determination of a PVT solution.
Although the multiple acquisition and tracking channels may increase the cost of the GPS receiver, they may reduce the so-called “time to first fix” (TTFF), which is the time required to provide a first PVT solution, typically, after the GPS device is initially powered.
FIG. 1 is a block diagram of a typical GPS receiver 100. The GPS receiver 100 may include an antenna 101, a radio frequency (RF) section 105, a digital section 115 and a processor 150. The antenna 101 is coupled to the RF section 105. The antenna 101 receives RF signals and provides those RF signals to the RF section 105. The RF signals may include, among other signals, GPS signals transmitted from a GPS source, such as one or more GPS satellites. The RF section 105 may processes the RF signals and may produce an intermediate frequency (IF) signal. In one embodiment, the IF signal may be produced by sampling the RF signal with an analog-to-digital converter (ADC, not shown). The RF section 105 provides the IF signal to the digital section 115.
The digital section 115 may use the IF signal to acquire and track GPS satellites and produce acquisition and tracking data that may be coupled to the processor 150. The digital section 115 includes acquisition channels and tracking channels. In this exemplary embodiment, the digital section 115 includes acquisition channels 120A and 120B and tracking channels 130A and 130B. Other embodiments of the digital section 115 may include more than two acquisition channels and more than two tracking channels. Acquisition channels 120A and 120B may be used to determine if a particular GPS satellite signal may be acquired. As described above, generally, the greater the number of acquisition channels in a GPS receiver, the shorter the TTFF.
Multiple tracking channels enable the GPS receiver 100 to concurrently track multiple GPS satellites and thereby continue to provide a PVT solution especially when one or more GPS signals may temporarily fade or become blocked. For example, if a GPS receiver is tracking five GPS satellites and the GPS signals from one of the GPS satellites is lost, then the GPS receiver may still be able to determine a PVT solution.
The processor 150 receives acquisition and tracking data from the digital section 115 and may determine a PVT solution. The processor 150 may also configure and control the RF section 105 and the digital section 115. For example, the processor 150 may provide gain settings for the RF section 105 or may configure one or more acquisition channels 120A and 120B in the digital section 115.
There are some applications that may benefit from the information contained in GPS signals, but which may not require a full PVT solution. As is well-known, relatively accurate time of day information is carried in GPS signals (specifically in the time-of-week field and the week number field) making the GPS signals a desirable time source. However, increasing the cost of a timepiece with the cost of a GPS receiver may make the cost of a GPS-assisted timepiece unattractive. Furthermore, if the timepiece is located indoors, the relative strength of the GPS signal may be diminished by the walls and roof, thereby increasing the difficulty of using time of day information carried by the GPS signal.
As the foregoing illustrates, what is needed in the art is a high sensitivity GPS receiver that has the ability to extract time of day information from the GPS signal without necessarily determining a full PVT solution.